In a data network it is conventional for a network file server containing disk storage to service storage access requests from multiple network clients. The storage access requests, for example, are serviced in accordance with a network file access protocol such as the Network File System (NFS) protocol, or the Common Internet File System (CIFS) protocol. NFS is described, for example, in RFC 1094, Sun Microsystems, Inc., “NFS: Network File Systems Protocol Specification,” Mar. 1, 1989. The CIFS protocol is described, for example, in Paul L. Leach and Dilip C. Naik, “A Common Internet File System,” Microsoft Corporation, Dec. 19, 1997.
A network file server typically includes a digital computer for servicing storage access requests in accordance with at least one network file access protocol, and an array of disk drives. This server computer has been called by various names, such as a storage controller, a data mover, or a file server. The server computer typically performs client authentication, enforces client access rights to particular storage volumes, directories, or files, and maps directory and file names to allocated logical blocks of storage.
Due to the overhead associated with the network file access protocol, the server computer in the network file server may become a bottleneck to network storage access that is shared among a large number of network clients. One way of avoiding such a bottleneck is to use a network file server system having multiple server computers that provide concurrent access to the shared storage. The functions associated with file access are distributed among the server computers so that one computer may receive a client request for access to a specified file, authenticate the client and authorize access of the client to the specified file, and forward the request to another server computer that is responsible for management of exclusive access to a particular file system that includes the specified file. See, for example, Vahalia et al. U.S. Pat. No. 6,192,408 issued Feb. 20, 2001, incorporated herein by reference.
In a network file server system having multiple server computers that provide concurrent access to the shared storage, the server computers may exchange file data in addition to metadata associated with a client request for file access. For example, as described in Xu et al. U.S. Pat. No. 6,324,581 issued Nov. 27, 2001, incorporated herein by reference, each file system is assigned to a data mover computer that has primary responsibility for managing access to the file system. If a data mover computer receives a client request for access to a file in a file system to which access is managed by another data mover, then the secondary data mover that received the client request sends a metadata request to the primary data mover that manages access to the file system. In this situation, the secondary data mover functions as a Forwarder, and the primary file server functions as the Owner of the file system. The primary data mover responds by placing a lock on the file and returning metadata of the file to the secondary data mover. The secondary data mover uses the metadata to formulate a data access command for accessing the file data over a bypass data path that bypasses the primary data mover.
In practice, the method of Xu et al. U.S. Pat. No. 6,324,581 has been most useful for large input/output (I/O) operations. The method of Xu et al. U.S. Pat. No. 6,324,581 has been used commercially in the following manner. For a small I/O operation of less than a given threshold, for example four kilobytes, of data to be read or written to a file system in storage, then the data mover computer in the network file server that is responsible for managing access to the file system will access the requested data in the conventional fashion. In general, the threshold is smaller than the file system block size. For a larger I/O operation of more than the threshold, then the data mover in the network file server that is responsible for managing access to the file system will function as a metadata server as described in Xu et al. U.S. Pat. No. 6,324,581 by placing a lock on the file to be accessed and returning metadata so that the metadata can be used to formulate a read or write request for accessing the data of the file over a path that bypasses the data mover.